The present invention relates to an oscillatory densimeter and in particular to a method of detecting an oscillation period of an oscillating tube of the oscillatory densimeter on the basis of an oscillation signal obtained by driving the oscillating tube.
The oscillatory densimeter is an instrument to measure densities of liquids which fill the oscillating tube on the basis of the oscillation period of the tube, for the oscillating period is determined depending on the densities of the liquids.
FIG. 3 is a schematic diagram illustrating an essential part of the oscillatory densimeter. Oscillating tube 1 is a thin, U-shaped tube filled with a liquid R for which density is to be measured. A magnet 4 is attached to oscillating tube 1, at the bottom of the U-shape. The oscillation period of tube 1 is detected through magnet 4 by a detection head 2 arranged against the magnet. As initial oscillation of oscillating tube 1 is very weak and soon attenuates, an external force must be applied to tube 1 so that a signal indicative of the oscillation can be maintained strong enough to be measured until the detection is completed. A driving head 3 arranged adjacent to detection head 2 applies the external force thereto to drive oscillating tube 1 on the basis of oscillation signal S.sub.1 generated by detection head 2. Oscillation signal S.sub.1 is an approximate sine wave as illustrated in FIG. 4(a) and applied to driving head 3 after being amplified as it is or shaped into a rectangular pulse signal as illustrated in FIG. 4(b). However, accuracy of the measurement may be deteriorated when oscillation signal S.sub.1 is distorted or the phase thereof is not stable in comparison with a genuine sine wave.
Generally speaking, relation between density .rho.x of the liquids and oscillation period Tx is represented by an expression (1) ##EQU1## wherein .rho.x: density of the substance of which density is to be measured
.rho.A: density of a first reference substance A PA1 .rho.B: density of a second reference substance B PA1 Tx: oscillation period of the substance of which density is to be measured PA1 TA: oscillation period of the first reference substance A PA1 TB: oscillation period of the second reference substance B
This expression, however, is not always precisely applicable to the results obtained by the prior art as described above.